Alloreactive memory T cells present a serious hurdle in clinical transplantation. Antibody-mediated depletion is widely used as induction therapy in sensitized transplant recipients to overcome the deleterious effects of preexisting donor-reactive immunity. However, memory T cells are less susceptible to depletion than na?ve T cells and there are no current efforts to improve the efficacy of induction therapies in targeting pre-existing donor-reactive T cell memory. Our previous studies in a mouse model of cardiac transplantation using rabbit anti-murine thymoglobulin (mATG) showed that memory T cells are a dominant component of anti-donor immune responses following lymphoablation and that recovering memory CD8 T cells are the primary effector mechanism mediating allograft rejection in mATG treated recipients. Preliminary experiments identified two groups of signals driving memory CD8 T cell recovery following antibody-mediated depletion: help from CD4 T cells mediated through B cells and post-transplant inflammation. The goal of the proposed study is to determine the mechanisms of memory T cell reconstitution following antibody mediated depletion and to use this information to develop strategies inhibiting memory T cell recovery and improving allograft outcome in high risk recipients. We hypothesize that 1) interference with helper and pro-inflammatory signals will effectively inhibit memory CD8 T cell recovery and increase the Treg/Teff cell ratio to impede the development of pathogenic anti-donor responses, and 2) these clinically feasible strategies will improve the efficacy of lymphocyte depleting induction therapies in high risk recipients containing alloreactive memory T cells and receiving cadaveric donor organs with prolonged ischemia time. We will test this hypothesis in three Specific Aims: Aim 1. To determine cellular and molecular requirements for memory T cell reconstitution following antibody - mediated lymphoablation. Aim 2. To test the effects of post-transplant inflammation on memory T cell recovery and functions in ATG treated recipients. Aim 3. To test the contribution of regulatory T cells to allograft prolongation by lymphoablative strategies. We anticipate that the approaches developed in these studies will specifically target pre-existing donor-reactive memory T cells and will improve the efficacy of lymphoablation in sensitized transplant patients.